Retirement of DKRZs Mistral

[suessspeise]

The former DKRZ cluster Mistral is no longer running (See e.g. last sentence of this article). This makes the following catalogs outdated:

1. mistral-MPI-GE.json
2. mistral-cmip5.json
3. mistral-cmip6.json
4. mistral-miklip.json